Add socket path to scope["server"]

[Kludex]

I think we can refactor a bit what is here, but this should solve #2559.

[nggit]

I haven't tried it yet but those seem like the right changes, which are safe to do without much refactoring.

Only minor (subjective):

• /tmp is global and it's better to use a unique prefix for files like uvicorn- or uvicorn/, otherwise create test.sock in the current directory - Edit: But that's okay, if MockSocket thing doesn't create a real file
• if rather than elif.